This application is based on German Application DE 199 12 733.6, filed Mar. 20, 1999, which disclosure is incorporated herein by reference.
The invention relates to a process for producing hydrogen peroxide by direct synthesis. In the process hydrogen and oxygen are converted in the presence of a heterogeneous catalyst containing at least one noble metal, in particular palladium, as its catalytically active component, in the presence or absence of a solvent. Use of a catalyst according to the invention provides advantages over the use of previously known catalysts with regard to H2 selectivity and/or the attainable H2O2 concentration.
The direct synthesis of hydrogen peroxide by converting hydrogen and oxygen in an acidic aqueous medium in the presence of a noble metal supported catalyst is known from various references, for example, European Patent Disclosure EP-B 0 272 830. In the process described in this reference and also in the processes referred to therein, an aqueous reaction medium is used which, for the sake of inhibiting the decomposition of hydrogen peroxide formed contains a strong acid, such as H2SO4 and/or HCI. Pd on activated charcoal, or other heterogeneous catalysts containing Pd and/or Pt, catalyze the composition. By adding a bromide promoter, the selectivity is increased. This process has various problems, among them selectivity that is too low and/or too low an attainable H2O2 concentration, and/or a low space-time yield, and sometimes also a high catalyst discharge; as a result, the industrial expense required for recovering the catalyst increases, and the H2 selectivity drops as the service life of the catalyst increases.
Many references are accordingly directed to furnishing improved catalysts for this process. In the process of European Patent Disclosure EP-A 0 366 419, a gas mixture containing H2 and O2 is passed through a catalyst bed disposed in a trickle-bed reactor, while at the same time an aqueous phase containing H2SO4 and HCI is trickled in parallel flow over the catalyst bed. Although high selectivity is attained in this process by using a noble metal catalyst bonded to a hydrophobic support, under the usual pressure and temperature conditions, nevertheless the high selectivity is at the disadvantageous cost of a very low H2O2 concentration (0.15 to 0.3%). To obtain commercial H2O2 solutions, complicated concentration and/or distillation steps must follow, which thus reduce the economy of the process.
In the process according to Japanese Patent Disclosure JP-A 7-241473, with a conventional reaction sequence, hydrogen and oxygen are reacted in an acidic aqueous medium in the presence of catalyst particles with a particle size below 50 nm that are applied to hydrophobic support particles. The catalyst particles are gold particles, while the hydrophobic support particles are hydrophobic or hydrophobized organic and inorganic substances. The production of the hydrophobic support that contains gold particles includes precipitation of a basic gold salt onto the hydrophobic vehicle from an aqueous solution that contains HAuCI4 and a reduction and/or calcination step for converting the gold compound to elemental gold. In a trickle-bed reactor, a 4.8 wt. % aqueous hydrogen peroxide solution is obtained with 80% selectivity.
A further trickle-bed process is taught by EP-A 0 579 109. Here an aqueous phase in parallel flow with the gas mixture containing the H2 and O2 is trickled over a catalyst bed based on a noble metal that is bonded to either activated charcoal, an inorganic oxide, or a sulfate. It is essential to adhere to a certain volume ratio of the gas phase to the liquid phase. With good selectivity for H2, an aqueous hydrogen peroxide solution with a content of approximately 5 wt. % is obtained. An increase in the H2O2 concentration is successfully achieved in the process described in German Patent Application DE 198 16 297.9, in which a gas mixture containing hydrogen and oxygen, which is substantially saturated or supersaturated with water vapor, is delivered to the reactor, and the aqueous phase is only then formed by condensation. In this process, known catalysts are used, in particular those having one or more elements of Group VIII and/or Group I of the periodic system. The catalytically active elements are typically bonded to a particulate or monolithic support.
In the postprocessing of the example in EP-A 0 579 109, it has been demonstrated that if a catalyst based on 2 wt. % Pd on activated charcoal is used with the aqueous hydrogen peroxide, a high palladium discharge is brought about, which shortens the service life of the catalyst.
In German Patent Disclosure DE-A41 27 918, a palladium/gold alloy with a content of 5-95 wt. % gold on a solid support such as carbon or aluminum oxide is used as the catalyst for the described reaction, in order to obtain a high formation speed, high yield and high selectivity. The production of the catalyst requires a two-stage process. As has been demonstrated, in this case also there is a need to increase the H2 selectivity and the H2O2 concentration.
The object of the present invention is accordingly to provide an improved process of this general type that leads to higher H2 selectivity and/or to higher H2O2 concentration. In a preferred embodiment, ways are also to be shown in which the noble metal discharge from the reactor can be reduced and the aqueous hydrogen peroxide and thus the service life of the catalyst can be increased.
The invention comprises a process for producing hydrogen peroxide by direct synthesis, in which hydrogen and oxygen are converted in the presence of a heterogeneous catalyst containing at least one noble metal, in particular palladium, as its catalytically active component, in the presence or absence of a solvent. The catalyst used has a catalytically active component that substantially comprises palladium or at least two metals selected from the platinum group (Group VIII) and Group I of the Periodic Table of Elements and which has been produced by spray pyrolysis or flame pyrolysis, including the conversion of a solution or suspension, containing compounds of the one or more catalytically active metals in a suitable elemental ratio in dissolved form, into a gas-supported particle collective. introduction thereof into a spray pyrolysis or flame pyrolysis reactor, the compounds of the catalytically active metals being converted substantially into these metals or alloys thereof, and separation of the solid particles formed from the gas stream.
The process according to the invention can be performed using the catalytically active component, that is, the palladium or noble metal alloy obtained by spray or flame pyrolysis, or a combination of catalytically active components. As an alternative, and generally preferably, these catalytically active metals or alloys are used in supported form. The supports can be particulate materials, such as powders, extrudates, granulates, or other molded bodies made from a powdered material. For producing the supported catalysts that contain the catalytically active component or components, known processes can be employed. For instance, it is possible for the powder of the catalytically active component produced by spray pyrolysis or preferably by flame pyrolysis to be mixed with a powdered support material, plasticized, and deformed, and then for the molded bodies to be solidified by calcination. It is also possible for the metal powder to be mixed with a support or carrier and made into tablets. It is additionally possible for the powdered catalytic component or components to be dispersed in a suitable liquid and impregnated into an already prefabricated molded support. Then, depending on the type of procedure employed, either a catalyst largely saturated with metal or a so-called eggshell catalyst, in which the metal is located only in the outermost zone of the support, or even a so-called subsurface-shell catalyst, in which the nucleus and the outermost shell of the support are largely free of the catalytic component, can be obtained. The production of the supported catalysts from the support material and the catalytically active component or components can be carried out in the presence or absence of organic or inorganic binders.
Examples of suitable binders are water glass, calcium oxalate, boric acids, and other glass-forming compositions. Organic binders, although they are less suitable for the purpose in question, are for instance polyolefins and fluorocarbonated polymers and organosilanes. Good fixation of the catalytically active component to the support is also possible by calcination, with the calcination being expediently performed at a temperature above 200xc2x0 C., in particular at 300-600xc2x0 C., and especially preferably at 400-600xc2x0 C., By means of a calcining step and especially preferably by the use of binders, or a combination of these provisions, supported noble metal catalysts are obtained which, in the generic process, with the aqueous hydrogen peroxide, discharge substantially less noble metal than was the case with the use of many previously known catalysts, and preferably discharge no noble metals at all.
The production of the catalytically active component containing at least one noble metal by the so-called spray pyrolysis technique or the flame pyrolysis technique is known in this field. For instance, EP-A 0 591 881 discloses a process for producing fine noble metal powders by spray pyrolysis: A solution of a thermally decomposable noble metal compound, or a mixture of such compounds, in a volatile solvent is converted into an aerosol. This aerosol is treated in an inert carrier gas in a pyrolysis reactor, at a temperature above the temperature of decomposition of the noble metal compound or compounds and below the melting point of the noble metal or noble metal alloy. The finely powdered metal or metal alloy formed is then separated from the carrier gas. The pyrolysis reactor can also, according to EP-A 0 593 167, be operated at a temperature above the melting point of the noble metal or noble metal alloy.
In so-called flame pyrolysis, the aerosol of superfine droplets in solution or suspension is fed into a flame reactor, such as an oxyhydrogen gas flame according to the J. Chem. Soc. Japan (1987), Vol.12, pages 2293-2300, or into the flame of a flame reactor, such as in German Patent DE-PS 195 45 455, in which the hydrogen combusts with the ambient air The advantage of the noble metal powder obtained in a flame reactor is its very high uniformity with respect to the composition and its very uniform snape. A very narrow particle size spectrum is also obtained.
In a further embodiment of the process of the invention, supported noble metal catalysts are used in which not only the catalytically active component but also the support material, which is an oxidic or silicate material, are simultaneously obtained by flame pyrolysis. Such catalysts can be obtained in accordance with German Patent Disclosure DE-A 196 47 038 by providing that the solution to be converted into an aerosol contains not only one or more noble metal compounds but additionally one or more oxide- and/or silicate-forming precursors. In the pyrolysis reactor, oxidic or silicate carrier particles are formed from these precursors, with particles of noble metal or noble metal alloy superfinely distributed on and/or in the carrier particles. DE-A 196 47 038 is expressly incorporated by reference herein.
In a further alternative, suitable supported catalysts for the generic process can also be obtained by delivering an aerosol, having superfine droplets that contain not only one or more dissolved noble metal compounds but also the undissolved support material in superfine form, to the flame pyrolysis reactor or to a reactor heated by an electric plasma and/or inductive plasma. In this embodiment, the catalytically active noble metal particles settle on the surface of the carrier particles and are solidly fixed to the carrier particles. This embodiment is described in German Patent Application DE 198 21 144.9, and the content thereof is hereby expressly incorporated into the disclosure of the present application.
Besides palladium, which is known to be especially effective as a catalyst, the catalysts preferably include promoters selected from the group comprised of platinum, rhodium, iridium, copper, silver and gold. In a preferred embodiment of the process of the invention, a catalyst is used in which the catalytically active component is a palladium alloy, containing at least 80 wt. % of palladium and from 0.05 to 15 wt. % of gold and/or 0.05 to 15 wt. % of platinum and 0 to 5 wt. % of silver, possibly together with typical impurities. Especially suitable support materials are oxidic and silicate supports, in particular Al2O3, TiO2, ZrO2, SnO2, SiO2, Ce2O3 and zeolites.
As has been demonstrated by comparative tests, the catalysts to be used in accordance with the invention can be used to perform different but intrinsically known direct synthesis processes for producing hydrogen peroxide in the presence of an aqueous or alcohol (DE-A 196 42 770) phase. Analogously, the process can also be performed in accordance with International Patent Disclosure WO97/32812 in the absence of any solvent at all; the reaction gas mixture flowing through a fixed catalyst bed is unsaturated with water and hydrogen peroxide, and hydrogen peroxide formed is separated from the reaction gas in a downstream separation stage.
The advantages of the process of the invention are that it has become possible, by means of the particular selection of the catalyst, to increase the selectivity and/or the attainable hydrogen peroxide concentration significantly. This increase in effectiveness of the catalytic system was surprising, because the surface area of the catalytically active component, in the form in which it can be obtained by a spray pyrolysis or flame pyrolysis technique, is substantially less than that of conventional catalysts, such as those in which the production includes a precipitation step and a thermal and/or chemical reduction step.